Low sulfur motor gasoline (mogas) requires the production of low sulfur blend stocks for the mogas pool. The primary blend stocks are derived from cat cracked (e.g., FCC) naphthas, which in addition to unwanted organic sulfur compounds, are rich in olefins desirable for meeting high octane rating requirements. The naphtha is reacted with hydrogen in the presence of a sulfided hydrodesulfurization catalyst, which forms H2S and a sulfur-reduced naphtha. The H2S is separated from the sulfur-reduced naphtha, which is then used for mogas. Such catalysts are known and typically contain a catalytic component of at least one Group VIB metal, and more often a mixture of non-noble Group VIII and Group VIB metals. In addition to having catalytic activity for removing sulfur and other heteroatoms, these naphtha hydrodesulfurization catalysts have hydrogenation activity, which saturates the desirable olefins.
Selective one and two-stage hydrodesulfurization processes have been developed to avoid olefin saturation and concomitant octane loss. Some processes use catalysts that have been at least partially deactivated by coke formation or by the use of inhibitors in order to decrease the olefinic saturation that accompanies desulfurization. However, the partial deactivation also substantially reduces the sulfur removal activity of the catalyst, which is undesirable. Other naphtha processes use catalysts made with metal compounds, which selectively and permanently poison hydrogenation sites. The hydrogenation activity of these catalysts cannot be restored, even with regeneration. Such processes are disclosed, for example, in U.S. Pat. Nos. 5,286,373; 5,525,211; 5,423,975; 5,985,136 and 6,231,754. There is a need for a naphtha desulfurization process using a catalyst whose olefinic saturation (hydrogenation) activity has been selectively suppressed, without substantially reducing its hydrodesulfurization activity. A further process improvement would result if this selective suppression could be achieved with the catalyst on-line in a reactor. Such one-line, selective suppression could be accomplished without the need for taking the reactor off-line, removing the catalyst, treating the catalyst, recharging the reactor, and then restarting the naphtha desulfurization process.